Getting Clojure

Deeply Recursive

Along with letting you do interesting things with your function arguments, Clojure also provides specialized support for writing recursive functions. A recursive function is, of course, a function that calls itself. For example, suppose we had this collection of book maps:

(def books
  [{:title "Jaws"  :copies-sold 2000000}
   {:title "Emma"  :copies-sold 3000000}
   {:title "2001"  :copies-sold 4000000}])

and we wanted to know the total number of books sold. We could write a recursive function to run through all the elements of the vector:

(defn sum-copies
  ([books] (sum-copies books 0))
  ([books total]
    (if (empty? books)
      total
      (sum-copies
        (rest books)
        (+ total (:copies-sold (first books)))))))

Notice ...